Systems and devices herein generally relate to printing devices and more particularly to printing devices that dry printed sheets.
Many times printers use marking material (e.g., inks, etc.) that transfer moisture (e.g., water or other materials that are in liquid form at internal operating temperatures of the printing device) into the printed sheets of media (such as paper, card stock, transparencies, etc.), and such moisture can be removed to prevent color non-uniformity, wrinkling, papers adhering to one another, paper jams, etc. For example, a cutsheet aqueous inkjet printer may dry printed sheets before output to prevent offset between the transport elements, sheet-to-sheet adhesion in a stacker, cockle (e.g., wrinkling) due to a water residing too long within the sheet, etc.
When drying sheets immediately after printing, a sufficient amount of heat is provided to raise the water temperature in the aqueous ink, while avoiding ignition of the printed sheets during a paper jam. The drying goals are first to remove the water that has been added by printing, second to prevent over-drying the plain non-jetted sheet (e.g., maintaining the overall relative humidity (Rh) to below a specified percentage, such as below 3%), third to maintain the internal humidity in the dryer cavity at a desirable percentage (e.g., 6% to 8%.) with different ambient Rh conditions, and fourth to remove the bulk moisture from the printer to the exterior of the building without creating a load on the HVAC of the building.
For example, a printed sheet drying system that uses a perforated vacuum belt results in relatively large conductivity of air near the perforations, whereas the other areas of the belt that are in direct contact with the sheet provide a much lower relative heat conductivity. These differences in conductivity can produce a thermal gradient, which will create different boundary conditions for drying and could result in low media weight prints having visible drying mottle on the backside of the page. Thus, if impingement holes are used to dry they could create mottle and non-uniformity on the front of the page.
Further, it is beneficial if the large air handling requirements of the drying system avoid disturbing the paper on the transport as it is moving under the dryer, so as to maintain expected sheet positions and arrival times.